Walter Dean Myers, whose realistic portrayal of the struggles of youths in the city made him a best-selling children’s book author, and who was an evangelist for literacy and education, died on Tuesday in Manhattan. He was 76.

His publisher, HarperCollins, said he died at Beth Israel Medical Center in Manhattan after a brief illness.

A onetime troublemaker who dropped out of high school, Mr. Myers spent much of his adult life writing realistic and accessible stories about crime, war and life in the streets. He wrote more than 100 books, including “Monster” and “Lockdown,” and was the rare author to have a wide following among middle-school boys.

He was a three-time National Book Award nominee, received five Coretta Scott King awards for African-American fiction and from 2012 to 2013 served as national ambassador for young people’s literature, a position created in part by the Library of Congress.

Well before that, he traveled the country, visiting schools and prisons and libraries.

“He wrote with heart and he spoke to teens in a language they understood,” Susan Katz, president and publisher of HarperCollins Children’s Books, said in a statement.

In March, he wrote a cover essay in the Sunday Review section of The New York Times with the headline, “Where Are the People of Color in Children’s Books?” In it, he lamented the scarcity of characters from different racial, ethnic and economic backgrounds.

“Books transmit values,” he wrote. “They explore our common humanity. What is the message when some children are not represented in those books? Where are the future white personnel managers going to get their ideas of people of color? Where are the future white loan officers and future white politicians going to get their knowledge of people of color? Where are black children going to get a sense of who they are and what they can be?”

Mr. Myers’s books were usually narrated by teenagers trying to make the right choices when the wrong ones were so much easier. There was the 17-year-old hiding from the police in “Dope Sick,” or the boarding school student in “The Beast” who learns his girlfriend is hooked on drugs.

He was careful not to make judgments, and in the crime story “Monster” he left doubt over whether the narrator was really guilty.

The fourth of five siblings, he was born Walter Milton Myers in Martinsburg, W.Va., in 1937. His mother died when he was 18 months old, and his father, George, sent Walter to live with his first wife, Florence Dean, a cleaning woman and factory worker, and her husband, Herbert Dean, who raised him in Harlem. He took the pen name Walter Dean Myers in honor of his foster parents.

Over 6 feet tall by middle school, he was a basketball star, but also a stutterer who was teased often and fought back in return. At home, he was happy to sit quietly and read.

“There were two very distinct voices going on in my head, and I moved easily between them,” Mr. Myers wrote in his memoir, “Bad Boy,” which came out in 2001. “One had to do with sports, street life and establishing myself as a male.” The other voice, he said, “the one I had from my street friends and teammates, was increasingly dealing with the vocabulary of literature.”

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Mr. Myers was gifted enough to be accepted to one of Manhattan’s best public high schools, Stuyvesant. But he was also shy, too poor to afford new clothes and unable to keep up with the work. He began skipping school for weeks at a time and never graduated.

“I know what falling off the cliff means,” he told The Associated Press in 2011. “I know from being considered a very bright kid to being considered like a moron and dropping out of school.”

He enlisted in the Army on his 17th birthday and served three years, after which he was employed as a factory worker, a messenger on Wall Street and a construction worker. Anxious to be a writer, he contributed to Alfred Hitchcock’s mystery magazine and numerous sports publications. When his half brother Wayne was killed in Vietnam, he wrote a tribute for Essence magazine.

His first book — “Where Does the Day Go?” — was published in 1969 after he won a children’s literature contest for minorities. His visits with students and inmates not only gave him the chance to help others straighten out their lives; it also inspired some of his work.

“Lockdown,” a National Book Award finalist, began after Mr. Myers met a youth who was afraid to get out of jail, thinking he would only get in trouble again. For “Monster,” he remembered a boy who would talk about the crimes he committed in the third person, as if someone else had committed them.

“Then I found out that all the guys could do that,” Mr. Myers told The Associated Press. “They could separate themselves from their crimes. We come up with strategies for dealing with our lives, and my strategy might be different because my life has been different.”

Mr. Myers’s novel “On a Clear Day” is scheduled to come out in September.

His survivors include his wife, Constance, and two sons, Christopher and Michael. A daughter, Karen, died.